public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);

        if (savedInstanceState == null) {
            FragmentManager fm = getSupportFragmentManager();

            fm.beginTransaction().replace(R.id.main_fragment, MainFragment.newInstance()).commitNow();
            fm.beginTransaction().replace(R.id.ad_fragment, AdMobFragment.newInstance()).commitNow();
        }
    }
}

public class MainFragment extends Fragment {

    private MainViewModel mViewModel;

    public static MainFragment newInstance() {
        return new MainFragment();
    }

/*    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.main_fragment, container, false);
    }
*/
    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        mViewModel = new ViewModelProvider(this).get(MainViewModel.class);
        // TODO: Use the ViewModel
    }

}

public class AdMobFragment extends Fragment {
    private FragmentAdMobBinding binding;
    public static AdMobFragment newInstance() {
        return new AdMobFragment();
    }

    //Use RequestConfiguration.Builder().setTestDeviceIds(Arrays.asList("534153A818F863B61501C7071FB0D8BA"))
    private void setContainerHeight(FragmentActivity view, int id, int dp) {
        ConstraintLayout layout = view.findViewById(id);
        ViewGroup.LayoutParams layoutParams = layout.getLayoutParams();

        layoutParams.height = dp;
        layout.setLayoutParams(layoutParams);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater,
                             @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {

        binding = FragmentAdMobBinding.inflate(inflater, container, false);
        FragmentActivity activity = getActivity();
        MobileAds.initialize(activity, new OnInitializationCompleteListener() {
            @Override
            public void onInitializationComplete(@NonNull InitializationStatus initializationStatus) {}
        });

        //AdRequest adRequest = new AdRequest.Builder().addTestDevice(AdRequest.DEVICE_ID_EMULATOR).build();
        AdView adView = activity.findViewById(R.id.ad_view);
        AdSize adSize;

        DisplayMetrics displayMetrics = new DisplayMetrics();
        getActivity().getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);

        adSize = AdSize.getCurrentOrientationAnchoredAdaptiveBannerAdSize(activity, (int) (displayMetrics.widthPixels/displayMetrics.density));

        setContainerHeight(activity, R.id.ad_fragment, adSize.getHeight());
//        setContainerHeight(activity, R.id.main_fragment, -adSize.getHeight());
        adView.setAdSize(adSize);
        //adView.loadAd(adRequest);
        return binding.getRoot();
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        AdView adView = binding.adView;
        AdRequest adRequest = new AdRequest.Builder().build();
        adView.loadAd(adRequest);
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <androidx.constraintlayout.widget.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/main_fragment"
        android:layout_width="match_parent"
        android:layout_height="100dp"
        android:background="#FF00FF00"
        tools:context=".ui.main.MainFragment">

        <TextView
            android:id="@+id/message"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Main Fragment"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

    </androidx.constraintlayout.widget.ConstraintLayout>

    <androidx.constraintlayout.widget.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:background="#FFFF0000"
        android:id="@+id/ad_fragment"
        tools:context=".ui.main.AdMobFragment">

        <!-- view for AdMob Banner Ad -->
        <com.google.android.gms.ads.AdView
            android:id="@+id/ad_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_centerHorizontal="true"
            app:adUnitId="ca-app-pub-3940256099942544/6300978111"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent" />

    </androidx.constraintlayout.widget.ConstraintLayout>

</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/ad_fragment"
    tools:context=".ui.main.AdMobFragment">

    <!--Change to ButtonImage with default ad-->
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="why?"
        app:layout_constraintBottom_toTopOf="@id/ad_view"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <!-- view for AdMob Banner Ad -->
    <com.google.android.gms.ads.AdView
        android:id="@+id/ad_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        app:adSize="BANNER"
        app:adUnitId="ca-app-pub-3940256099942544/6300978111"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>